Summary The long-term goal of the proposed research is a paradigm shift towards limb size management rather than socket size management in prosthesis users experiencing socket fit changes during the day. The focus of this application is to implement and optimize a novel limb-volume management strategy, and then characterize its impact on prosthesis user clinical outcomes, health, and quality-of-life. The specific aims of the proposed research are to monitor user activity and socket fit within prostheses equipped with novel socket release/relock mechanisms. Socket release is the relief of limb-socket pressures during sitting, and relock is the return to the normal socket configuration. Different release/relock operations conducted while sitting and their influence on subsequent ambulatory limb mechanics, socket fit, and functional outcomes are tested. The procedure most beneficial to patient outcomes is implemented in long-term testing. Participants wear prostheses with the socket release/relock mechanism in their free-living environments for four weeks. Outcome measures are compared with results from traditional sockets. To accomplish the aims, unobtrusive sensors that monitor prosthesis use and limb-socket distances indicative of socket fit are integrated into prostheses with motor-driven socket release/relock mechanisms. A repeated- measures study is conducted using a standardized protocol to determine if the ordering of pin relock and socket relock after socket release affects subsequent limb fluid volume retention, limb vertical and angular positions in the socket, user comfort, and functional performance. A crossover study is conducted to determine if use of release/relock sockets compared with traditional sockets for 4 weeks enhances participant activity, socket fit, self-reported outcomes, energy level, and residual limb health. The relevance of the proposed application to public health is better understanding how use of active release/relock affects users? experience with a prosthesis, clinically-important outcomes related to residual limb health, and users? performance in life activities. Currently, it is unknown to what degree active release/relock benefits prosthesis users? quality of life. Potentially, the efforts described here can be extended to other areas where devices require a secure and intimate fit with human tissues despite changes in soft tissue volume, including trans-fermoral prosthetics, upper-limb prosthetics, and orthotics.